Common Mode Transient Immunity (CMTI) is one of the most important key parameters of an isolator. CMTI describes the ability of an isolation barrier to withstand fast common mode transients applied between two isolated circuits and thus to maintain the system integrity. To measure the CMTI, very fast voltage changes between the two isolated circuit grounds must be generated. This paper presents a novel method to generate the transients and measure the CMTI. A DC-DC converter is used with a high voltage switch in addition to an appropriate regulation circuit using only passive elements to generate the desired voltage level and slope. The method was implemented on an evaluation board to evaluate a few commercial digital isolators. Measurement results are presented and discussed.